The present invention relates to a method for the fire-inhibiting impregnation of wood, preferably in the form of thin wood flakes or sheets, e.g. thin veneers or wood chips.
In the course of time, much effort has been directed to solving the problem of imparting fire-inhibiting properties to wood, as well as resistance to the growth of fungi, attack by termites and moisture. Efforts have been made in particular with a view to imparting fire-inhibiting properties to wood. The basic prior art comprises an impregnation of the wood with fire-inhibiting salts that are applied in aqueous solution. For example, ammonium sulphate, sodium sulphate or magnesium sulphate, monoammonium phosphate or diammonium phosphate, borates or the like can be used. The fire-inhibiting or flame-proofing effect of such salts may be based on the fact that their decomposition is endothermic and that on being heated they easily form salts which envelop the inflammable substance, so that same is unable to give off combustible gases and carbonizes without any flames being formed.
Normally, no difficulty is encountered in impregnating wood with the salts mentioned, which easily penetrate into the pores and intermicellar cavities of the wood. On account of the solubility in water of these salts, however, they are rather easily washed out of the wood again, and not only if the wood comes into direct contact with water, for instance with rain, but also if the wood is exposed to moisture in some other way, such as the natural humidity in the atmosphere, condensate or similar moisture occurrences. The result is that even if the impregnated wood in the beginning is resistant to fire, it subsequently becomes just as flammable as nonimpregnated wood. It has been tried to use a large excess of salt, up to 30% by weight, but no significant improvements have been obtained thereby, and, moreover, this is uneconomical. Attempts to mix the salts with other agents, e.g. polyethylene glycol, have also failed to give entirely satisfactory results.